The present invention relates to elevator technology and, more particularly, to friction sheaves for flat ropes having variable shoulder and crowning elements for controlled rope-sheave engagement.
Certain elevator traction drive systems utilize flexible, flat ropes for various advantages over traditional round-sectioned ropes. In addition to several performance and durability advantages, there are certain disadvantages associated with flexible flat ropes. Among them, is the potential for belt tracking problems.
Tracking problems can lead to a variety of performance problems. For example, improper belt tracking can lead to undesirable noise which may be discomforting to passengers in an elevator car who perceive the noise as an indication of an unsafe condition. Improper tracking may affect the smoothness of the elevator car ride. Extended improper tracking may result in premature wear and may require premature replacement of elevator belts.
It is an object of the present invention to provide a sheave design that minimizes or eliminates improper belt tracking in an elevator system.
It is a further object of the present invention to overcome the shortcomings attributable to conventional and known elevator sheave designs such as those described above.
These and other objects are achieved by the present invention in the manner described below.
Testing has shown that sheaves made from a single material, such as steel or cast iron, can be prone to tracking problems with flexible flat ropes. For instance, when the belt is off-center with regard to a belt groove the belt may rub on the shoulder of the belt groove. This condition leads to undesirable noise and eventual degradation of the belt. The rate of belt degradation along the side of the belt groove is dependent on the level of sheave misalignment as well as on the material interface between the belt and the sheave materials. Typical flexible flat ropes comprise a jacket made of a flexible material such as urethane. While the urethane jacket engages the bottom of the sheave belt groove with a desired amount of friction, it also has a tendency to engage the groove shoulder when misaligned, causing undesirable frictional engagement with the shoulder.
The amount of crowning on the bottom of the sheave groove is a factor affecting belt tracking. Excessive crowning, forming a convex surface in profile, promotes belt degradation due to unequal stress gradients in the flexible flat rope during operation. On the other hand, insufficient crowning can result in the belt being undesirably sensitive to sheave misalignments.
The traction capacity of a drive sheave is an important factor in the design and operation of an elevator system. As the desired traction dictates the material selection and surface finish of a sheave groove, the surrounding surfaces, such as the groove shoulders, are the same as the belt-contact surface of the groove in conventional sheave designs. This is due to the use of single material and uni-body construction for some components of conventional sheaves.
The present invention is directed to a segmented elevator sheave assembly having independently variable shoulder and crowning elements for use with flexible flat ropes in an elevator system. This enables selection of high-traction surface finish and crowning for the sheave groove that contacts the belt, and of low-traction, smooth sheave groove shoulders having a low friction coefficient that may inadvertently contact the belt side surfaces. A center hub mounted to two bearings for concentric rotation provides means for keeping the segments of the sheave in a concentric relationship. The present invention design makes the system more forgiving to angular misalignment of the sheave and reduces the likelihood of premature degradation of the flat flexible rope due side-surface scuffing along sheave groove shoulders.